は・
In relaxed conversation people leave out は, が and を when it is clear without them who is doing what. Keeping them in is never a mistake; among friends it can sound a little careful.
How it works
Noun + は／が／を + … → Noun + …
Only the particle goes. The noun stays, often with a small pause after it, written as a comma.
Also
Place + に／へ + 行く・来る・帰る・着く → Place + 行く…
で, と, から, まで, の and も stay.
を, は and が
The verb and the situation say what the noun is doing there. Bread in front of "eat" can only be the thing eaten, so nobody misses を.
Short polite sentences do it too. You have probably said this one already.
Going somewhere
A place just before 行く, 来る, 帰る or 着く can lose its に or へ. Other uses of に mostly stay: a time (三時に), a person you give something to (母にあげる).
Where are you going?
Are you going to school tomorrow?
Contact me when you get to the station.
The ones that stay
The other particles say something the verb can't supply: who with, what by, where from. Take one out and the sentence means something else, or nothing.
My friend went.
Now the friend is the one who went: this is が dropped.
Not said: バス行く？
(Are you going by bus?)
It has to be バスで行く？.
When は and が are said
Among friends the bare noun is the plain way to say it, so a は that is said out loud can be heard as a contrast.
I'm not going.
It can sound like: the rest of you can go, but not me.
Where が picks someone out, as in だれが and its answer, it usually stays.
Who is coming?
Where it doesn't belong
In writing, apart from messages to friends, and in a speech, an interview or a presentation. There every particle goes in.
Close neighbours
はN5
Names what you are talking about, or sets one thing against another. The second kind usually stays.
I drink coffee, but I do not drink tea.
が・けどN5
After a whole sentence, が means "but". That one doesn't drop.
It is already nine, but the store is still open.
Check yourself
1You are talking to a friend. Which of these can you say?
Choose an answer to see why.
2You are writing a report for your teacher. Which fits?
Choose an answer to see why.